Mask film to form relief images and method of use

ABSTRACT

A mask-forming film has a transparent layer between the imageable layer and the carrier sheet, which transparent layer has a refractive index that is lower (by at least 0.04) than that of the carrier sheet or any immediately adjacent layer between it and the carrier sheet. This lower refractive index layer modifies the path of incident radiation during mask image transfer so as to provide steeper shoulder angles in the relief image solid areas.

FIELD OF THE INVENTION

This invention relates to a film or element in which a mask image can beformed that then can be used to form an imaged element bearing a reliefimage. In particular, this invention relates to mask-forming films thatare readily useful for preparing flexographic printing plates, and tomethods of making such plates.

BACKGROUND OF THE INVENTION

Radiation-sensitive elements having a laser-ablatable mask layer on thesurface are known in the art. A relief image can be produced in suchelements without the use of a digital negative image or other imagedelement or masking device. Films with a laser-ablatable mask layer canbe formed by first imagewise exposing the film with laser radiation(generally an infrared radiation laser under computer control) toselectively remove the mask layer in the exposed areas. The masking filmis then placed in contact with a radiation-sensitive element andsubjected to overall exposure with actinic radiation (for example, UVradiation) to cure the radiation sensitive element in the unmasked areasand thus form a negative image of the mask in the element. The filmcontaining the mask layer and the imaged radiation-sensitive element(such as an imaged printing plate precursor) are then subjected tosolvent development. The unexposed printing plate areas and the masklayer are completely developed off, and after drying, the resultingimaged element is useful, for example as a flexographic printing plate.

While flexographic printing plates having an integral laser-ablatablemask layer allow direct imagewise exposure using a laser and do notrequire a separate masking device, the time for imaging is generally toolong since the system sensitivity to imaging radiation is low. Variousattempts have been made in the industry to overcome this problem byincreasing the infrared sensitivity of the mask layer. However,obtaining higher sensitivity has been a challenge due to the widelyvarying quality criteria that must be simultaneously satisfied. Inaddition, this approach requires the use of high-powered laser-equippedimaging apparatus that is especially configured for imaging flexographicarticles. Because of the need for varying the thickness of flexographicplates depending upon the specific intended uses, more than one imagingapparatus may be required for the integral-mask approach.

An important advance in the art of making and using masking films isdescribed in U.S. Patent Application Publication 2005/0227182 (Ali etal., hereinafter cited as US '182). The described method provides a maskimage in significantly less time due to greater imaging sensitivity.

Problem to be Solved

Although the method of relief image formation, as described in US '182provides a mask image in significantly less time, it has been observedthat when UV exposure is done through the carrier sheet of the maskfilm, the resulting shoulder angle is lower than desirable. This resultsin a higher level of halation of the printed image. The higher level ofhalation is particularly noticeable at higher impression pressure duringprinting.

For printing, surface quality and properties of a flexographic printingplate are important attributes. In practice, prolonged exposure time isoften necessary in order to hold or fully cure the smaller features suchas the high-light dots, for example, 1% to 5% dots (where percent refersto the amount of paper covered with print ink) of high quality printimages. However, the over-exposure fills in the reverse lines or shadowareas. Thus, over-exposure results in image quality degradation.

The term “exposure latitude” describes the degree to which aphotosensitive element can be over-exposed with only negligible imagequality degradation. Exposure latitude can be further defined as theability to simultaneously image low light throughput features, forexample, 1-2% dots, and high light throughput features, for example,4-mil reverse lines, onto a flexographic plate. Photosensitiveflexographic printing plates with larger exposure latitude are desirableas they are more tolerant to the actual exposure time used during frontimage-wise exposure and are thus easier to use.

Halation in flexographic printing is a well known. U.S. Pat. No.6,864,039 (Cheng Lap Kin et al.) describes halation caused by scatteringof the UV light within non-imaged areas of the photopolymerizablemedium. As nearly all heterogeneous photocrosslinkable compositionsexhibit some degree of light scattering, prolonged image-wise exposureleads to a high level of background scattered actinic radiation, whichis often sufficient to cause cross-linking or curing of polymer inregions not exposed to imagewise radiation. The overall effect of suchunwanted cross-linking is the filling-in of fine negative that is“halos”, around solid image areas. Halos lead to degradation in theprint quality of flexographic printing plates and are linked to dot-gainthat is the formation of a larger image dot size than intended. Thispatent discloses the use of photobleachable compounds in thephotopolymer composition to increase resistance to scattered light.

U.S. Pat. No. 5,496,685 (Farber et al.) also describes halation caused IS by excessive scattering or irregular reflection of light from thesupport of the printing element, resulting in shallow relief. It alsoteaches the use of an actinic radiation absorber to improve exposurelatitude.

EP 0 504 824A (Swatton et al.) describes the use of antihalation agentsin the support of the photopolymer. The antihalation agents are actinicradiation absorbers.

Another cause of halation is the presence of low-angle-of incidenceradiation during exposure that can enter the photopolymer below the maskat the edges of exposed areas, reducing the shoulder angles. As theaverage shoulder angle decreases below 50°, the loss in the reliefsharpness becomes increasingly noticeable and as the average shoulderangle decreases below 40° there is a considerable loss in printstability and sharpness. A collimated light source may reduce thehalation by reducing the level of lower angle incident light. However, acollimated light source is more expensive to use than commonly usedpoint light or bank light source.

Adding a low refractive index antihalation layer to photographic silverhalide films to control unwanted incident or scattered light isdescribed in U.S. Pat. No. 2,481,770 (Nadeau). But the use of suchlayers in masking films to provide flexographic printing plates isunknown.

There is a need to solve the problem caused by lower angle incidentradiation in the preparation of relief images in imageable elements suchas flexographic printing plate precursors so the relief imagepredominantly has shoulder angles of at least 50°. There is also a needto improve exposure latitude so that small dots can be retained on aplate without degrading the shadow images and reverse lines.

SUMMARY OF THE INVENTION

To address these problems, the present invention provides a filmcomprising a transparent carrier sheet and having thereon at least onenon-silver halide thermally sensitive imageable layer and a colorantdisposed in the imageable layer or in a different layer between theimageable layer and the carrier sheet,

an infrared radiation absorbing compound dispersed in a polymericbinder, and

the film further comprising a transparent layer disposed between thecarrier sheet and the imageable layer and between the carrier sheet andthe layer containing the colorant, which transparent layer has arefractive index lower than the refractive index of the carrier sheet orthe refractive index of any optional immediately adjacent layer betweenthe carrier sheet and the transparent layer.

This invention also provides a method of making a relief imagecomprising:

A) forming a mask image by forming exposed and non-exposed regions in animaged film, which film prior to imaging, comprises:

a transparent carrier sheet having thereon at least one non-silverhalide thermally sensitive imageable layer and a colorant disposed inthe imageable layer or in a different layer between the carrier sheetand the imageable layer,

an infrared radiation absorbing compound dispersed in a polymericbinder, and

a transparent layer disposed between the carrier sheet and the imageablelayer, which transparent layer has a refractive index lower than therefractive index of the carrier sheet or the refractive index of anyoptional immediately adjacent layer between the carrier sheet and thetransparent layer,

B) transferring (for example, by laminating) the mask image in theimaged film to a radiation-sensitive element while there is completeoptical contact between the mask image and the radiation-sensitiveelement,

C) exposing the radiation-sensitive element to curing radiation throughthe carrier sheet and the mask image to form an imaged element, whereinthe mask image is opaque to the curing radiation, and

D) developing the imaged element to form a relief image.

In some embodiments, a method of making a relief image uses a film ofthis invention that comprises on the carrier sheet, in order:

a) the transparent layer comprising a fluoroelastomer and having athickness of from about 0.2 to about 10 μm, the transparent layeroptionally including an adhesion promoter,

b) an intermediate layer comprising a poly(vinyl alcohol) and having athickness of from about 0.2 to about 10 μm, and optionally including anadhesion promoter,

c) a barrier layer comprising a poly(cyanoacrylate) and an infraredradiation absorbing dye,

d) the imageable layer comprising an infrared radiation absorbing dyeand a UV-absorbing colorant dispersed in a binder, and

e) an overcoat layer, comprising a methacrylic acid copolymer andfluoropolymer particles,

the transparent layer having a refractive index at least 0.04 lower thanthe refractive index of the carrier sheet.

The present invention provides an improved mask-forming film and methodof using it to provide imaged radiation-sensitive elements (such asflexographic printing plates) with improved relief images thatpredominantly have shoulder angles of at least 50° while holding desiredsmall dot features in halftone areas and maintaining good reverse linedepth.

When the imaged film (masking film) is used to form a relief image in aradiation-sensitive element, the imaged film is placed into intimate orcomplete optical contact with the element in such a manner as toeliminate any air, void space, or gap at the interface (thus, an“air-free” interface). Such a gap may be under vacuum so that air islacking, but such a gap under vacuum also would not be considered an“air-free” interface.

The unique film of the present invention also provides a modified pathfor incident relief-forming radiation at the air-free interface duringexposure through the mask so that incident radiation light is properlybent to enter the radiation-sensitive element at a desired angle toprovide a steeper shoulder angles or slopes around the edges of imagedareas in the resulting relief image.

The unique film of the present invention is believed to restrict theentry of low-angle-of-incidence radiation into the radiation-sensitivematerial by the process of total internal reflection at the interfacebetween the high refractive index carrier sheet and the low refractiveindex transparent layer. The angle of incidence (φ_(critical)) abovewhich total internal reflection occurs is dependent on the differencebetween the high refractive index carrier sheet and the refractive indexof the transparent layer can be calculated by Snell's law and is givenexplicitly by the relationship φ_(critical)=arc sin(R_(L)/R_(H)),wherein R_(L) is the refractive index of the low-index medium and R_(H)is that of the high-index medium.

These improvements are achieved by incorporating a transparent layerwith a lower refractive index in the film of this invention between itsimageable layer and the carrier sheet. The incorporated transparentlayer has a lower refractive index than the carrier sheet or anyoptional immediately adjacent layer that is in direct contact with thetransparent layer on its carrier sheet side.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a cross-sectional representative schematic view of a solidraised printing area in a relief image having a “steep” shoulder angle,for example that is at least 50°.

FIG. 1 b is a cross-sectional representative schematic view of a solidraised printing area in a relief image having a “shallow” shoulderangle, for example that is less than 50°.

FIG. 2 is a graphical representation of relief height (μm) versusdistance from the solid edge (μm) obtained in Invention Examples 1 and 2and Comparative Example 1.

FIGS. 3 a and 3 b are cross-sectional views of actual micrographicimages of relief images provided by Invention Example 1 and ComparativeExample 1 described below.

FIG. 4 is a schematic diagram illustrating the definition of “shoulderangle” (0) for this invention.

DETAILED DESCRIPTION OF THE INVENTION Definitions:

Unless otherwise indicated, the “film” described herein is an embodimentof the present invention. The film may also be known as a “maskelement”, “mask film”, or “masking element”. Upon imaging, the film maybe known as a “mask”, “imaged film”, or “imaged masking film” andcontains a “mask image”.

Unless otherwise indicated, percentages are by weight.

The term “radiation-sensitive element” used herein includes anyimageable element or material in which a relief image can be produced byexposure through the imaged masking film. Examples ofradiation-sensitive elements include, but are not limited, toflexographic printing plate precursors, printed circuit boards, andlithographic printing plate precursors.

By “ablative”, we mean that the imageable layer of the film can beimaged using a thermal ablating means such as laser radiation thatcauses rapid local changes in the imageable layer thereby causing thematerial(s) in the imageable layer to be ejected from the layer. This isdistinguishable from other material transfer or imaging techniques inthat a chemical change rather than a physical change (for example,melting, evaporation, or sublimation) is the predominant mechanism ofimaging.

By “optical contact” we mean that two layers or two elements (as in thecase of the imaged masking film and a radiation-sensitive element) arein intimate contact so that there is essentially no air-gap or voidbetween the contacted surfaces, thus providing an “air-free interface”.More precisely, two surfaces are defined as being in optical contactwhen the reflection and transmission characteristics of their interfaceare essentially fully described by the Fresnel laws for the reflectionand transmission of light at the refractive-index boundary.

“Shoulder angle” is the angle defined by the flat printing surface andthe slope of the edge of the raised area, as illustrated for example, inFIG.4. A “predominant shoulder angle of at least 50°” means that withina given area of the relief image, the average shoulder angle of solidedges is at least 50° and preferably at least 55°. By “average shoulderangle” we mean the average angle of slope of the edge-wall down to adepth of 100 micrometers from the printing surface.

Film:

The film of this invention is used to form a mask image used eventuallyto form a relief image. This film comprises two or more layers,including one or more imageable layers and a transparent (low refractiveindex) layer disposed on a transparent carrier sheet. The film mayinclude one or more other layers including one or more of a barrierlayer, intermediate or intermediate layer, adhesive layer, or otherlayers generally used in the art in masking films according to US '182noted above. Different constructions of the film may be used in one ormore different imaging methods.

Carrier sheet:

The carrier sheet may be any suitable transparent substrate. Usefulcarrier sheets include but not limited to, transparent polymeric filmsand sheets such as polyesters including poly(ethylene terephthalate),poly(ethylene naphthalate), and fluorine polyester polymers,polyethylene, polypropylene, polybutadienes, polycarbonates,polyacrylates, polyvinyl chloride and copolymers thereof, and hydrolyzedand non-hydrolyzed cellulose acetate. Generally, the carrier sheet isfrom about 20 to about 200 μm thick. For example, a transparentpoly(ethylene terephthalate) sheet sold under the name of MELINEX byDuPont Teijin Films (Hopewell, Va.) is suitable for this purpose.

If necessary, the carrier sheet surface can be treated to modify itswettability and adhesion to applied coatings. Such surface treatmentsinclude but are not limited to corona discharge treatment and theapplication of subbing layers.

In addition, the carrier sheet can contain one or more “adhesionpromoters” that improve adhesion between the carrier sheet and the nextadjacent layer, whatever type of layer or whatever purpose that layermay have. Useful adhesion promoters include but are not limited to,gelatin, polyvinylidene chloride, poly (acrylonitrile-co-vinylidenechloride-co-acrylic acid), and polyethylenimine.

Transparent Layer:

The transparent layer is generally comprised of one or more film-formingpolymeric materials that collectively provide a refractive index that islower than the refractive index of the carrier sheet (or any optionalimmediately adjacent layer between the transparent layer and the carriersheet). This difference in refractive index may be as low as 0.04 andmore typically at least 0.08. One skilled in the art can readilydetermine useful polymeric film-forming materials since there arehundreds of possible materials commercially available. To see if a givenmaterial is useful, its refractive index (if not already known from theart or trade literature), can be determined by, for example, preciselymeasuring the location of the interference maxima in the spectral scanof a thin, uniform film of the material over the required range ofwavelengths. This refractive index can then be compared to that of thecarrier sheet (or optional intermediate layer) whose refractive index isknown in the art or can be determined using a known procedure such asthat just described.

By “transparent”, we mean that the transparent layer that is generallyhas a transmission optical density of less than 0.3, and is thus notconsidered opaque or even translucent. The transparent layer desirablyhas a refractive index that is at least 0.08 lower than the refractiveindex of the supporting film base.

Certain classes of film forming polymeric materials that can be usedinclude one or more fluoroelastomers such as those described in U.S.Pat. No. 5,176,972 (Bloom et al.). Such polymers include fluorinatedacrylate polymers that are derived from fluorinated acrylate monomersthat have the following formula:

CH₂═CR—C(═O)—O—(CH₂)_(n)—Y-T

wherein n is 1 or 2, R is hydrogen or a methyl group, Y is aperfluoroalkylene group, and T is a fluorine or —CF₂H group (forexample, 1H,1H-pentadeca-fluoroocyl acrylate, 1H,1H,5H-octafluoropentaylacrylate, trifluoroethyl acrylate, and heptafluorobutyl acrylate). Thefluorinated monofunctional acrylate monomer may also contain heteroatomssuch as oxygen, sulfur, and nitrogen atoms, for example having thefollowing formula:

CH₂═CR—C(═O)—O—(CH₂)₂—NR′—SO₂-Z

wherein Z is H(CF₂)_(m) or F(CF₂)_(m) wherein m is an integer of from 3to 12, R is hydrogen or a methyl group, and R′ is an alkyl group.

Such monomers or polymers derived therefrom can be obtained from anumber of commercial sources including 3M Corporation (St. Paul, Minn.).

Other useful fluoroelastomers include fluoroolefins such as copolymersof vinylidene fluoride and hexafluoropropylene, terpolymers ofvinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene,mixtures of two or more of such polymers, or blend of such copolymers orterpolymers with polytetrafluoroethylene (PTFE) that can be provided asa latex. Some of these fluoroelastomers can be obtained from 3MCorporation, or they can be prepared by copolymerization of knownmonomers using known conditions as described for example in U.S. Pat.No. 5,176,972 (noted above). One specific copolymer of this type isavailable from 3M Corporation as Fluorel FC-2175.

Alternatively, the transparent layer can be composed of film-formingpolymeric materials that do not of themselves have the desiredrefractive index, but various non-film-forming materials such as mattingagents, fillers, microcapsules, or a salt, can be dispersed in thefilm-forming materials that act as binders, to provide the desiredrefractive index. Examples of such dispersed additives, are described inU.S. Pat. No. 2,481,700 (Kuan-han Sun et al.) that include but are notlimited to, NaBF₄ and NH₄BF₄ dispersed in poly(vinyl alcohol), andMgSiF₆ dispersed in a suitable binder.

The transparent layer generally has a substantially uniform thickness ofat least 0.25 μm and typically from about 0.4 to about 10 μm. It isgenerally provided as a substantially uniform coating with littlevariation in thickness over its entire area.

The transparent layer can also contain adhesion promoters in addition toor alternative to the carrier sheet. Examples of useful adhesionpromoters are polyethylenimine, poly(vinylidene chloride), and similarcopolymers, and Fusabond (sold by Dupont). Adhesion promoters are chosensuch that they are soluble in the coating solvent for the low refractiveindex material.

Imageable Layer(s):

The imageable layer(s) are generally disposed on the transparent layeras relatively uniform coatings (that is, being substantially continuousand having fairly uniform thickness). In some embodiments, the imageablelayer and transparent layer underneath it are the only layers on thecarrier sheet. In other embodiments, there are multiple layers includingmultiple imaging layers or an imageable layer with a barrier layer,intermediate, or other layer(s) as described below.

The components of the imageable layer(s) are chosen to be soluble orswellable in suitable flexographic printing plate developers includingboth chlorinated organic solvents and the non-chlorinated organicsolvents described below that are used to create the relief image afterexposure of the radiation-sensitive element to curable radiation throughthe imaged masking film.

The imageable layer(s) generally includes one or more “colorants” orsubstances that may or may not impart visible color based on totalsolids of the layer. The colorant is generally capable of strongabsorbance of the curing radiation or is otherwise capable of blockingcuring radiation. As used herein, “colorant” indicates a component thatsubstantially prevents the transmission of curing radiation through themask image.

The colorant may be one or more dyes or pigments, or mixtures thereofthat will provide desired spectral properties. It can be a particulatematerial that is dispersed within the polymeric binder(s) describedbelow. For example, they can be black dyes or pigments such as carbonblack, metal oxides, and other materials described for example in US'182 (noted above), that is incorporated herein in its entirety. It isuseful that the pigments or dyes be substantially non-IR absorbing sothat imaging of the radiation-sensitive element is not adverselyaffected. For example, the colorant can absorb UV or visible radiation,and in many embodiments, the colorant is an UV-absorbing dye.

In one embodiment, the colorant is a black dye or pigment that absorbsenergy at substantially all wavelengths across the visible spectrum, forexample from about 350 to about 750 nm. The black dye or pigment may bea mixture of dyes or pigments, or mixtures of both dyes and pigmentsthat individually may or may not be black but when mixed togetherprovide a neutral black color. For example, a mixture of NEPTUN Black,Blue Shade Magenta, and Red Shade Yellow pigment (available from BASF inGermany) that provide a neutral black color may be used. DISPERSAL CBJ(from Runnemade Dispersions KV of the UK) may also be suitable.

One suitable black pigment is carbon black of which there are numeroustypes with various particles sizes that are commercially available.Examples include RAVEN 450, 760 ULTRA, 890, 1020, 1250 and others thatare available from Columbian Chemicals Co. (Atlanta, Ga.) as well asBLACK PEARLS 170, BLACK PEARLS 480, VULCAN XC72, BLACK PEARLS 1100 andothers available from Cabot Corp. (Walthan, Mass.).

The colorant(s) can be present in the imageable layer in an amount offrom about 10 to about 50 weight %, and typically from about 10 to about40 weight %.

It may be desirable to combine the use of carbon black with anon-infrared absorbing black dye or pigment to reduce interference withthe radiation and improve the quality of the resulting imaged maskingfilm. Also suitable as a pigment is a non-carbonaceous particulatematerial such as metal particles or metal oxide particles.

The imageable layer(s) generally also includes one or more infraredradiation absorbing compounds. In some embodiments, the colorant acts inthis function also but in other embodiments, a separate compound isincluded for this purpose, that is, to sensitize the imageable layer(s)to imaging IR radiation. Thus, the infrared radiation absorbing compoundis sensitive to radiation in the range of from about 700 to about 1500nm and typically from about 700 to about 1200 nm. Examples of useful IRabsorbing compounds include but are not limited to, cyanine infraredradiation (IR) absorbing dyes, carbon blacks, and metals such asaluminum. In one embodiment, a mixture of IR dyes is used, which IR dyescan absorb at different wavelengths, for example at 830 nm and 1064 nm.

Examples of suitable IR dyes include but are not limited to, azo dyes,squarilium dyes, croconate dyes, triarylamine dyes, thiazolium dyes,indolium dyes, oxonol dyes, oxaxolium dyes, cyanine dyes, merocyaninedyes, phthalocyanine dyes, indocyanine dyes, indotricarbocyanine dyes,oxatricarbocyanine dyes, thiocyanine dyes, thiatricarbocyanine dyes,merocyanine dyes, cryptocyanine dyes, naphthalocyanine dyes, polyanilinedyes, polypyrrole dyes, polythiophene dyes, chalcogenopyryloarylideneand bi(chalcogenopyrylo)polymethine dyes, oxyindolizine dyes, pyryliumdyes, pyrazoline azo dyes, oxazine dyes, naphthoquinone dyes,anthraquinone dyes, quinoneimine dyes, methine dyes, arylmethine dyes,squarine dyes, oxazole dyes, croconine dyes, porphyrin dyes, and anysubstituted or ionic form of the preceding dye classes. Suitable dyesare also described in U.S. Pat. No. 5,208,135 (Patel et al.), U.S. Pat.No. 6,569,603 (Furukawa), and U.S. Pat. No. 6,787,281 (Tao et al.), andEP Publication 1,182,033 (Fijimaki et al.). A general description of oneclass of suitable cyanine dyes is shown by the formula in paragraph[0026] of WO 2004/101280, incorporated herein by reference.

In addition to low molecular weight IR-absorbing dyes, IR dye moietiesbonded to polymers can be used as well. Moreover, IR dye cations can beused as well, that is, the cation is the IR absorbing portion of the dyesalt that ionically interacts with a polymer comprising carboxy, sulfo,phospho, or phosphono groups in the side chains.

Near infrared absorbing cyanine dyes are also useful and are describedfor example in U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No.6,264,920 (Achilefu et al.), U.S. Pat. No. 6,153,356 (Urano et al.),U.S. Pat. No. 5,496,903 (Watanate et al.). Suitable dyes may be formedusing conventional methods and starting materials or obtained fromvarious commercial sources including American Dye Source (Baie D'Urfe,Quebec, Canada) and FEW Chemicals (Germany). Other useful dyes for nearinfrared diode laser beams are described, for example, in U.S. Pat. No.4,973,572 (DeBoer).

Useful IR absorbing compounds include carbon blacks, some of which aresurface-functionalized with solubilizing groups are well known in theart. Carbon blacks that are grafted to hydrophilic, nonionic polymers,such as FX-GE-003 (manufactured by Nippon Shokubai), or which aresurface-functionalized with anionic groups, such as CAB-O-JET® 200 orCAB-O-JET® 300 (manufactured by the Cabot Corporation) are also useful.

The radiation absorbing compound (for example IR-absorbing compound) isgenerally present in an amount to provide a transmission optical densityof at least 0.5 and typically of at least 0.75 at the exposingwavelength. Generally, this is achieved by including from about 1 toabout 20 weight % of the one or more compounds, based on the solidscontent of the imageable layer. For example, the IR absorbing compoundshould be sufficient to produce transparent areas where the film isexposed to infrared radiation, meaning that such areas would have atransmission optical density of about 0.5 or less as measured using asuitable filter on a conventional densitometer.

In other embodiments, the radiation absorbing compound may include anultraviolet absorber that absorbs radiation at from about 150 to about400 nm. The UV absorber can be used as the only radiation absorbingcompound or in combination with an IR absorber compound.

The imageable layer can optionally include a fluorocarbon additive forenhancing transfer of a molten or softened film and production ofhalftone dots (that is, pixels) having well-defined, generallycontinuous, and relatively sharp edges. Examples of useful fluorocarbonadditives and amounts are provided in [0087] to [0089] of US '182 (notedabove).

Additional optional components of the imageable layer(s) include but arenot limited to, plasticizers, coating aids or surfactants, dispersingaids, UV absorbers, and fillers, all of which are well known in the artas described for example in [0094] to [0096] of US '182 (noted above).

All of the components described above for the imageable layer(s) aredispersed in one or more polymeric binders (both synthetic and naturallyoccurring polymeric materials) that are capable of dissolving ordispersing the other components in the imageable layer. The one or morepolymeric binders are generally present in an amount of from about 25 toabout 75 weight %, and typically from about 35 to about 65 weight %,based on the total dry weight of the imageable layer.

While a wide variety of polymeric binders can be used, some advantagescan be achieved by using certain “primary” polymeric binders in anamount of at least 50 weight %, and typically at least 70 weight % andup to 100 weight %, of the total polymeric binder weight. The usefulpolymeric binders are those into which various components can beincorporated and that are soluble in suitable coating solvents such aslower alcohols, ketones, ethers, hydrocarbons, and haloalkanes. Thepolymeric binders are also desirably soluble or swellable in the chosendeveloper (described below).

Polymeric Binders:

Useful polymeric binders include the materials described for example in[0081] to [0085] of US '182, which paragraphs are incorporated herein byreference. The polymeric binders may be known as “adhesive binders” asdescribed for example in [0081] of US '182 (noted above). Examples ofsuitable adhesive binders include but are not limited to, acetylpolymers such as poly(vinyl butyral)s that can be obtained for exampleas BUTVAR® B-76 from Solution, Inc. (St. Louis, Mo.) and acrylamidepolymers that can be obtained as MACROMELT 6900 from Henkel Corp. (GulphMills, Pa.). Pressure-sensitive adhesive polymers may also be used forthis purpose.

In some embodiment, where the colorant layer is ablatable, it isadvantageous to use binders that are easily thermally combustible, andgenerates gases and volatile fragments at temperature less than 200° C.Examples of these binders are nitrocellulose, polycarbonates,polyurethanes, polyesters, polyorthoesters, polyacetals, and copolymersthereof (see U.S. Pat. No. 5,171,650 of Ellis et al., Col. 9, lines41-50).

Other useful secondary polymeric binders are resins having hydroxylgroups (or hydroxylic polymers) as described in [0082] to [0084] of US'182 (noted above) and include for example poly(vinyl alcohol)s andcellulosic polymers (such as nitrocellulose). Still further secondarypolymeric binders are non-crosslinkable polyesters, polyamides,polycarbamates, polyolefins, polystyrenes, polyethers, polyvinyl ethers,polyvinyl esters, and polyacrylates and polymethacrylates having alkylgroups with 1 and 2 carbon atoms.

Some other useful polymeric binders that have been found to be readilydissolved or dispersed in non-chlorinated organic solvents are describedbelow. They may also be dissolvable or dispersible in chlorinatedorganic solvents also. Such useful classes of polymeric binders thatmeet these characteristics include but are not limited to, terpeneresins, phenolic resins, aromatic hydrocarbon resins, polyurethanes(including polyether polyurethanes), long-chain acrylate andmethacrylate resins. Useful terpene resins include but are not limitedto the SYLVARES terpene resins such as SYLVARES TR-A25 terpene resinthat is available from Arizona Chemical Co. (Jacksonville, Fla.). Usefulphenolic resins include but are not limited to, novolac resins such asCK2500 and CK2400 novolac resins that are available from Georgia PacificResins (Atlanta, Ga.). Aromatic hydrocarbon resins include but are notlimited to, NORSOLENE® resins such as NORSOLENE® S-155 resin that areavailable from Sartomer Co. (Warrington, Pa.). Useful polyurethanesinclude but are not limited to, SURKOPAK® 5245 and SURKOFILM® 72Spolyurethane resins that are available from Tennants Inks & CoatingsSupplies, Ltd. (Surrey, UK) and NeoRez 322 polyurethane resin that isavailable from DSM NeoResins (Wilmington, Mass.). Long chain acrylateand methacrylate resins include those vinyl polymers derived from one ormore long chain acrylate or methacrylate monomers wherein the long alkylchain has at least 3 carbon atoms. Such monomers include but are notlimited to, iso-butyl methacrylate, n-butyl methacrylate, and mixturesthereof.

Useful polymeric binders are homopolymers and copolymers derived from atleast iso-butyl methacrylate, n-butyl methacrylate, or mixtures thereof.Commercially available primary polymeric materials of this type includeELVACITE® 2045 and ELVACITE® 2046 polymers that are available fromLucite International (Cordova, Tenn.). For example, it was found thatthe commercial polymers available as SURKOPAK® 5245 polyurethane resinand SURKOFILM® 72S polyurethane resin, ELVACITE® 2045 polymericmaterial, and CK 2500 novolac resin are useful.

The imageable layer can further include plasticizers, coating aids,dispersing agents, UV absorbers, fillers, surfactants, fluorocarbons,and other additives as described in US '182 (noted above). Adhesionpromoters, such as those described above for the carrier sheet, can alsobe included.

Intermediate Layer:

In some embodiments of this invention, the film can include an“intermediate layer” disposed between the carrier sheet and theimageable layer(s). In some instances, the intermediate layer isdirectly disposed on the carrier sheet and between it and thetransparent layer described above. In other embodiments, theintermediate layer is directly disposed onto the transparent layer andis between it and the imageable layer. The presence of an intermediatelayer may be desirable to facilitate the transfer of a resulting maskimage to the radiation-sensitive element. Generally, the intermediatelayer is developable, dispersible, or easily removable after curingthrough the mask image or during subsequent processing (development) ofthe imaged element. Furthermore, the intermediate layer usually does notsignificantly absorb or scatter the curing radiation. For example, itusually does not include matte agents or other light scatteringmaterials. If a coating solvent is used to coat the intermediate layer,the coating solvent is chosen such that during coating there is littleintermixing between the transparent layer and the intermediate layer.

Representative coatings suitable for use as an intermediate layerinclude but are not limited to poly(vinyl alcohols) or similar polymers,cellulosic polymers such as methyl cellulose or hydroxypropyl methylcellulose, poly(vinyl butyral), or hydrolyzed styrene maleic anhydride.In this invention, UV exposure is carried out through the carrier sheetso that the carrier sheet prevents oxygen migration.

The intermediate layer may be relatively thin and have a dry thicknessof from about 0.1 to 10 μm.

In other embodiments, the intermediate layer is a thermally resistantpolymer layer that has desired layer integrity and good releaseproperties after thermal imaging. Thermally resistant polymers includebut are not limited to, polyimides, polysulfones, polyether ethylketone, bisphenol-A terephthalate, poly(vinyl alcohols), and polyamides,and can be optimized to provide desirable release properties,curability, and developability.

The intermediate layer may also include crosslinking agents to improverelease properties, coating aids, surfactants, and release-enhancingagents. Further details of useful intermediate layer compositions areprovided in US '182 (noted above).

Barrier Layer:

The film of this invention can also include a barrier layer disposedbetween the transparent layer or intermediate layer (if present) and theimageable layer(s) that may be used to prevent colorant migration intoor thermal damage to the transparent or intermediate layers during theablation process. In most embodiments, the barrier layer is disposedover the intermediate layer and under an imageable layer. Suitablebarrier layers and their compositions are also described in US '182(noted above) and references cited therein. For example, the barrierlayer may include one or more polymer binders, particularly,“heat-combustible” polymer binders such as poly(alkyl cyanoacrylate)sand nitrocellulose, and particulate materials such as metal oxideparticles (for example, iron oxide particles) to provide high opticaldensity with respect to imaging or curing radiation. Metal oxideparticles may be useful for ablative imaging because they can thermallydecompose to generate propulsive gases.

The barrier layer may optionally include an infrared absorbing compound,such as infrared absorbing dyes (IR dyes) including cationicinfrared-absorbing dyes and photothermal-bleachable dyes, andcrosslinking agents such as melamine-formaldehyde resins, dialdehydes,phenolics, polyfunctional aziridines, isocyanates, and urea-formaldehydeepoxies to provide greater thermal resistance.

Other Film Components:

An adhesive layer can be present in the film overlying the imageablelayer(s) to enhance adhesion of the mask image to theradiation-sensitive element during transfer and to aid in the transferof the mask image. The adhesive layer may comprise a thermoplastic,thermal adhesive, or pressure-sensitive adhesive that is well known inthe art.

In some embodiments the adhesive layer, or also used as an overcoatlayer, may comprise a methacrylic acid copolymer (such as a copolymer ofethyl methacrylate and methacrylic acid) and particles of one or morefluoropolymers dispersed therein as described, for example, in Example 1(top particle layer) in U.S. Pat. No. 6,259,465 (Tutt et al.). Theovercoat layer may also provide abrasion resistance to handling due tothe presence of the particulates. The overcoat layer may also act as adye barrier layer to prevent dye migration from the masking film to thephotopolymer after lamination.

Thus, in some embodiments of this invention, the film comprises acarrier sheet, having disposed thereon, in order:

a) a transparent layer as described above,

b) an intermediate layer as described above,

c) a barrier layer as described above,

d) an imageable layer as described above, and

e) an overcoat as described above,

wherein the transparent layer has a refractive index at least 0.08 lowerthan the refractive index of the carrier sheet.

Radiation-Sensitive Elements

Considerable details of useful radiation-sensitive elements such asflexographic printing plate precursors, printed circuit boards, andlithographic printing plates are provided in US '182 (noted above). Suchelements include a suitable dimensionally stable substrate, at least oneradiation-sensitive layer, and optionally a separation layer, coversheet, or metal layer. Suitable substrates include dimensionally stablepolymeric films and aluminum sheets. Polyester films are preferred. Anyradiation-sensitive element that is capable of producing a relief imageusing the film described herein is useful in the practice of thisinvention.

The radiation-sensitive element can be positive- or negative-working,but typically, it is negative-working and generally includes a visible-or UV-sensitive imageable layer containing a visible-radiation orUV-radiation curable composition that is cured or hardened bypolymerization or crosslinking upon exposure to the curing radiation.For example, the radiation-sensitive element can be UV-sensitive. Manydetails of various components of the radiation-sensitive elements areprovided in US '182 (noted above), and references cited therein.

Some embodiments also include a removable cover sheet as well as aseparation layer, or sometimes referred to as anti-tack layer, thathelps removal of the cover sheet and protects the radiation-sensitiveimageable layer from fingerprints and other damage and that is disposedbetween the radiation-sensitive imageable layer and the cover sheet.Useful separation layer materials include but are not limited to,polyamides, poly(vinyl alcohols), copolymers of ethylene and vinylacetate, amphoteric interpolymers, cellulosic polymers, poly(vinylbutyral), cyclic rubbers, and combinations thereof.

The radiation-sensitive imageable layer can include an elastomericbinder, at least one monomer, and an initiator that is sensitive tonon-IR radiation. In most cases, the initiator will be sensitive to UVor visible radiation. Suitable initiator compositions include but arenot limited to those described in U.S. Pat. No. 4,323,637 (Chen et al.),U.S. Pat. No. 4,427,749 (Gruetzmacher et al.), and U.S. Pat. No.4,894,315 (Feinberg et al.).

The elastomeric binder can be a single or mixture of polymers that maybe soluble, swellable, or dispersible in aqueous, semi-aqueous, ororganic solvent developers and include but are not limited to, bindersthat are soluble, swellable, or dispersible in organic solvents such asnatural or synthetic polymers of conjugated diolefins, block copolymers,core-shell microgels, and blends of microgels and preformedmacromolecular polymers. The elastomeric binder can comprise at least65% of the imageable layer based on total layer solids. More details ofsuch elastomeric binders are provided in [0190] of US '182 (noted above)and references cited therein.

The imageable layer can also include a single monomer or mixture ofmonomers that must be compatible with the elastomeric binder to theextent that a clear, non-cloudy radiation-sensitive layer is produced.Monomers for this purpose are well known the art and includeethylenically unsaturated polymerizable compounds having relatively lowmolecular weight (generally less than 30,000 Daltons). Examples ofsuitable monomers include various mono- and polyacrylates, acrylatederivatives of isocyanates, esters, and epoxides. Specific monomers aredescribed in [0191] of US '182 (noted above) and in references citedtherein.

The photoinitiator may be a single compound or combination of compoundsthat are sensitive to visible or UV radiation and that generate freeradicals that initiate the polymerization of the monomer(s) withoutexcessive termination and are generally present in an amount of fromabout 0.001 to about 10% based on the total dry weight of the imageablelayer. Examples of suitable initiators include substituted orunsubstituted polynuclear quinines and further details are provided in[0192] of US '182 (noted above) and in references cited therein.

The radiation-sensitive layer can include other addenda that providevarious properties including but not limited to sensitizers,plasticizers, rheology modifiers, thermal polymerization inhibitors,tackifiers, colorants, antioxidants, antiozonants, and fillers.

The thickness of the radiation-sensitive imageable layer may varydepending upon the type of imaged plate desired. In some embodiments, aUV-sensitive imageable layer may be from about 500 to about 6400 μm inthickness.

In one embodiment, the radiation-sensitive element is a flexographicprinting plate precursor that includes a suitable UV-curable resin andwhen exposed and processed, provides a flexographic printing plate. Suchelements generally include a suitable substrate, one or moreUV-sensitive imageable layers comprising a photosensitive material thatinclude a polymer or prepolymer. Examples of commercially availableflexographic printing plate precursors include but are not limited to,FLEXCEL flexographic elements available from Kodak Polychrome Graphics,a subsidiary of Eastman Kodak Company (Norwalk, Conn.), CYREL®Flexographic plates available from DuPont (Wilmington, Del.), NYLOFLEX®FAR 284 plates available from BASF (Germany), FLEXILIGHT CBU plateavailable from Macdermid (Denver, Colo.), and ASAHI AFP XDI availablefrom Asahi Kasei (Japan).

The radiation-sensitive element may also be used to form printed circuitboard wherein a conducting layer (also known as a “printing circuit) isformed on a substrate in the pattern dictated by the mask image.Suitable precursors to printed circuit boards generally comprise asubstrate, a metal layer, and a photosensitive layer. Suitablesubstrates include polyimide films, glass-filled epoxy orphenol-formaldehyde or any other insulating materials known in the art.The metal layer covering the substrate is generally a conductive metalsuch as copper or an alloy or metals. The photosensitive layer mayinclude an UV-curable resin, monomers, or oligomers, photoinitiators,and a binder. The photosensitive layer in the printed circuit boardprecursor may be a positive- or negative-working layer. Further detailsof printed circuit boards are provided in [0196] to [0205] of US '182(noted above)

Forming a Mask Image:

In the practice of this invention, a mask image is formed by producingexposed and non-exposed regions in the film of this invention. Thechoice of imaging mechanism will determine the possible variations informing the mask image, as described below.

Exposing the film can be carried out in selected regions, otherwiseknown as “imagewise exposure”. Both analog and digital methods can beused for imagewise exposure and are conventional in the art. In someembodiments, imagewise exposure can be accomplished using laserradiation from a laser that is scanned or rasterized under computercontrol. Any of the known scanning devices can be used includingflat-bed scanners, external drum scanners, and internal drum scanners.In these devices, the film is secured to the drum or bed, and the laserbeam is focused to a spot that can impinge on the film. Two or morelasers may scan different regions of the film simultaneously.

For example, the film can be exposed to infrared radiation, for example,in the range of from about 700 to about 1400 nm. Such films contain oneor more infrared radiation absorbing compounds as described above toprovide sensitivity to infrared radiation. In these embodiments, thefilm may be suitably mounted to an infrared imager and exposed to theinfrared radiation using an infrared laser such as a diode laser orNd:YAG laser that may be scanned under computer control. Suitableinfrared imagers include but are not limited to DESERTCAT 88 imagersavailable from ECRM (Tewksbury, Mass.) used in color proofing,TRENDSETTER imagesetters and ThermoFlex Flexographic CTP imagersavailable from Eastman Kodak Company (Burnaby, British Columbia, Canada)used for CTP lithographic plate applications and for imagingflexographic elements, DIMENSION imagesetters available from Presstek(Hudson, N.H.) useful for CTP lithographic plate applications, CYREL®Digital Imager (CDI SPARK) available from Esko-Graphics (Kennesaw, Ga.),and OMNISETTER imagers available from Misomex International (Hudson,N.H.) useful for imaging flexographic elements.

In other embodiments, the film is exposed to visible laser light, forexample in the range of from about 400 to about 750 nm. Commerciallyavailable filmsetters and imagesetters can be used including but notlimited to, ACCUSET Plus imagesetter (visible red laser diode, 670 nm)and ADVANTAGE DL3850 imagesetter (410 nm), SELECTSET 5000 imagesetter(HeNe, 630 nm), all available from Agfa-Gevaert (Belgium), LUXEL V-9600(410 nm) available from Fuji Photo Film (Japan), and DIAMONDSETTERimagesetter (frequency-doubled Nd-YAG laser, 532 nm) available fromWestern Lithotech (St. Louis, Mo.).

In still other embodiments, the film can be exposed to ultravioletradiation by laser direct imaging in the range of from about 150 toabout 410 nm. Apparatus useful for such imaging include but are notlimited to, DP-100 imagers available from Orbotech (Billerica, Mass.)and DIGIRITE 2000 imager available from Etec Systems (Tucson, Ariz.).

The step of forming the mask image may also include a step of removingeither exposed or non-exposed regions of imageable layer. In someembodiments, the exposed regions are removed, leaving a mask image onthe transparent carrier sheet (and transparent layer disposed thereon).For these embodiments, a receptor sheet may optionally be used forremoval of unwanted portions of the imageable layer. Such a receptorsheet may be any suitable paper, transparent film, or metal sheet towhich one or more coatings have been applied before irradiation of thefilms to facilitate transfer of the imageable layer to the receptor.After imaging, the receptor sheet may be removed from the film to revealthe mask image on the carrier sheet. A complementary image to the maskimage may remain on the receptor sheet.

In other embodiments, a mask image is formed on the carrier sheet (andtransparent layer disposed thereon) by producing exposed and non-exposedregions of the imageable layer and other layers, and removingnon-exposed regions of those layers.

In some embodiments, the mask image residing on the carrier sheet may becured by subjecting it to heat treatment, provided that the transferproperty of the mask image is not adversely affected. Heat treatment maybe done by a variety of means including but not limited to, storage inan oven, hot air treatment, or contact with a heated platen or passagethrough a heated roller device. Heat treatment is not necessary forcuring to take place.

In still other embodiments, a mask image can be formed as noted aboveand the exposed regions are transferred to a receptor sheet. Thereceptor sheet it then removed from the imaged masking film before themask image is transferred to a radiation-sensitive element. Thus, thefilm may be provided with a receptor sheet in contact with theradiation-sensitive element, or the element is contacted with a separatereceptor sheet.

Where a separate receptor sheet is used during imaging, the film andreceptor sheet are assembled in close proximity prior to imaging, withthe image-receiving side of the receptor sheet adjacent to the imageablelayer. The term “close proximity” in this context can mean that theimageable layer and receptor sheet are brought into contact, or thatthey do not contact each other but are sufficiently close to allowtransfer of imageable layer or colorant upon exposure to imagingradiation. Vacuum hold-down or a mechanical means may be used to securethe film and receptor sheet in assembly.

Next, the assembly of the film and receptor sheets is imagewise exposedusing imaging radiation to form a mask image, as described below.imagewise exposure causes imagewise transfer of imageable layer orcolorant from the film to the receptor sheet. After imaging, the filmmay be removed from the receptor sheet to reveal the mask image on thereceptor sheet.

Several imaging mechanisms are mentioned briefly below and furtherdetails are provided by US '182 (noted above) and references citedtherein beginning with paragraphs [0142].

Ablation:

In this mechanism, the exposed regions of the imageable layer areremoved from the imaged film by the generation of a gas, leaving a maskimage. Specific binders that decompose upon exposure to heat (such as IRlaser irradiation) to rapidly generate a gas may be used. This action isto be distinguished from other mass transfer techniques in that achemical rather than a physical change cases an almost complete transferof the imageable layer rather than a partial transfer.

Melt-Stick Technique:

The exposed areas of the imageable layer can be transferred in a moltenor semi-molten state from the imaged film to a suitable receptor sheetupon exposure to radiation. The exposed areas are characterized byreduced viscosity that provides flowability to the imageable layer thatflows across to and adheres to the surface of the receptor sheet withgreater strength than it adheres to the carrier sheet (and transparentlayer disposed thereon). Following this physical transfer, the carriersheet, along with the untransferred imageable layer, is separated fromthe receptor sheet.

In one embodiment, the mask image comprises the non-exposed regionsremaining on the carrier sheet. In another embodiment, the mask imagecomprises the exposed regions of the imageable layer that aretransferred to the receptor sheet.

Laser-Induced Film Transfer:

With this imaging mechanism, the exposed regions of the imageable layerare removed from the carrier sheet (and transparent layer disposedthereon) through laser-induced film transfer (“LIFT”). An intermediatelayer containing a latent crosslinking agent is disposed between thecarrier sheet and the imageable layer. The latent crosslinking agentreacts with the binder to form a high molecular weight network in theexposed regions to provide better control of melt flow phenomena,transfer of more cohesive material to the receptor sheet, and highquality edge sharpness of the mask image.

In one embodiment, the imageable layer includes a transferable colorantand an infrared absorbing dye (IR dye). In another embodiment, theimageable layer includes a transferable colorant, a polymeric binder asdescribed above, a fluorocarbon additive, a cationic IR dye, and latentcrosslinking agent as described above.

The mask image can comprise the non-exposed regions of the imageablelayer remaining in the imaged film, but in other embodiments, the maskimage comprises the exposed regions that are transferred to the receptorsheet.

Peel-Apart:

In this imaging mechanism, the exposed regions of the imageable layerare removed from the carrier sheet (and transparent layer disposedthereon) using a suitable receptor sheet based differential adhesionproperties in the imageable layer. After imagewise exposure of the film,the receptor sheet is separated from the carrier sheet and eitherexposed or non-exposed regions remain in the film.

Dye Sublimation or Diffusion:

In yet another imaging technique, colorant from exposed regions of theimageable layer is removed through sublimation wherein the colorant isdiffused or sublimed without simultaneous transfer of the binder. A maskimage may be generated in the film without the need for a receptorsheet. In other embodiments, a receptor sheet is used to capture thesublimed colorant. The mask image then comprises the imageable layerremaining in the imaged film. In still other embodiments, the mask imagecomprises the colorant that is transferred to a receptor sheet.

Alkaline Development of mask:

The exposed regions of the imageable layer can also be removed byconventional alkaline development when the imaged film is washed with asuitable alkaline developer while non-exposed regions remain on thecarrier sheet. The imageable layer is positive-working in this instanceand can be composed of any of the known positive-working compositions.The developer has a pH of from about 9 to about 14 and comprises waterand generally a hydroxide and other various addenda common to suchsolutions.

Alternatively, the non-exposed regions of the imageable layer areremoved from the imaged film to produce a mask image. Such imageablelayer compositions are negative-working and become insoluble in thedeveloper upon exposure. Useful developers for such materials generallyhave a pH of from about 7 to about 13 and include water-misciblehigh-boiling organic solvents and various addenda common to suchsolutions.

Useful developers for these materials are well known and available fromseveral sources including Eastman Kodak Company (Norwalk, Conn.).

Once the mask image has been formed, it is transferred to a suitableradiation-sensitive element (described above) that is sensitive tocuring radiation (usually UV radiation). Mask image transfer includesplacing the film with the mask image onto the radiation-sensitiveelement, or a radiation-sensitive composition or layer thereof.

The film and radiation-sensitive element are placed in such contact asto provide an air-free interface. Generally, this is achieved bylaminating the film it to the radiation-sensitive element by applyingpressure or heat, or both pressure and heat to form an air-free orgap-free interface.

Commercially available laminators that provide both heat and pressuremay be used including but not limited to, KODAK model 800XL APPROVALLAMINATOR available from Eastman Kodak Company (Rochester, N.Y.), CODORLPP650 LAMINATOR available from CODOR (Amsterdam, Holland), and LEDCO HDlaminators available from Filmsource (Casselbury, Fla.). A protectivecover sheet, if present in the film, is removed before lamination. Theassembled film with the mask image and radiation-sensitive element arefed into the laminator at the desired speed, temperature, and pressure.A representative example of this process is shown in the examples below.

In one embodiment, the radiation sensitive element for flexographicapplications do not have the separation layer (anti-tack layer), wherepressure alone may be sufficient to achieve air-free interface, as theradiation sensitive element is tacky, or acts as a pressure sensitiveadhesive, due to the presence of monomers.

In still another embodiment, transfer of the mask image can be achievedby using pressure-sensitive adhesion when the masking film andradiation-sensitive element are pressed into contact with each other toform an air-free interface. A pressure-sensitive adhesive may beincorporated into the radiation-sensitive element, or it may be placedin a separate layer between the imageable layer and theradiation-sensitive element. Suitable pressure-sensitive adhesives areknown in the art.

In still another embodiment, the mask image can be transferred usingwhat is known as a “liquid photopolymer process” in which aradiation-sensitive or photopolymer composition is uniformly applied, inliquid or paste form, to the transparent layer of the imaged filmcontaining the mask image, for example, by placing theradiation-sensitive composition between the imaged film and atransparent support material that then becomes the “support” orsubstrate for the radiation-sensitive element (defined below). Forexample, the transparent support material can be a polymeric film asdescribed above for the radiation-sensitive elements.

Exposure of Radiation-Sensitive Element

After an air-free contact is made between the mask film and theradiation sensitive element as described above, the radiation-sensitiveelement is exposed to curing radiation through the film containing themask image to form an imaged element. In this step, the curing radiationis projected onto the radiation-sensitive element through the mask imagethat preferentially blocks some of the radiation. In unmasked regions,curing radiation will cause hardening or curing of theradiation-sensitive composition(s). The mask image should therefore besubstantially opaque to the exposing radiation, meaning that the maskimage should have a transmission optical density of 2 or more andpreferably 3 or more. The unmasked regions should be substantiallytransparent meaning that the unmasked regions of the radiation-sensitiveelement should have a transmission optical density of 0.5 or less,preferably 0.1 or less, and more preferably 0.05 or less. Transmissionoptical density can be measured using a suitable filter on adensitometer, for example, a MACBETH TR 927 densitometer.

Generally, exposure of the radiation-sensitive element through the filmcontaining the mask image is accomplished by floodwise exposure fromsuitable irradiation sources (for example, visible radiation or UVradiation). Exposure can be carried out in the presence of atmosphericoxygen. Exposure under vacuum is not necessary as air-free contact (oroptical contact) has already been made.

In the manufacture of a relief printing plate, such as a flexographicprinting plate, one side of the radiation-sensitive element is generallyfirst exposed to curing radiation through a transparent support (knownas “back exposure”) to prepare a thin, uniform cured layer on thesupport side of the element. The radiation-sensitive element is thenexposed to curing radiation through the film containing the mask image,thereby causing the radiation-sensitive composition to harden or cure inthe unmasked areas. Unexposed and uncured regions of theradiation-sensitive element are then removed by a developing process(described below), leaving the cured regions that define the reliefprinting surface. The back exposure could be performed either before orafter the air-free contact is made between the mask film and theradiation sensitive element.

The wavelength or range of wavelengths suitable as the curing radiationwill be dictated by the nature of the radiation-sensitive element. Insome embodiments, the curing radiation is ultraviolet radiation at awavelength of from about 340 to about 400 nm. Sources of visible or UVradiation for floodwise or overall exposure include but are not limitedto, carbon arcs, mercury-vapor arcs, fluorescent lamps, electron flashunits, and photographic flood lamps. UV radiation is particularly usefulfrom mercury-vapor lamps and more particularly sun lamps. RepresentativeUV radiation sources include SYLVANIA 350 BLACKLIGHT fluorescent lamp(FR 48T12/350 VL/VHO/180, 115 watts) that has a central emissionwavelength of about 354 nm that is available from Topbulb (East Chicago,Ind.), and BURGESS EXPOSURE FRAME, Model 5K-3343VSII with ADDALUX754-18017 lamp available from Burgess Industries, Inc. (Plymouth,Mass.).

Other suitable sources of UV radiation include platemakers that are ableto both expose the radiation-sensitive element to radiation and todevelop the element after radiation exposure. Examples of suitableplatemakers include but are not limited to, KELLEIGH MODEL 310PLATEMAKER available from Kelleigh Corporation (Trenton, N.J.) and theGPP500F PLATE PROCESSOR available from Global Asia Ltd. (Hong Kong).

The time for exposure through the mask image will depend upon the natureand thickness of the radiation-sensitive element and the source of theradiation. For example, in one of embodiment, a FLEXCEL-SRH plateprecursor available from Eastman Kodak Company (Norwalk, Conn.) may bemounted on a KELLEIGH MODEL 310 PLATEMAKER and back exposed to UV-Aradiation through the support for about 20 seconds to prepare a thin,uniform cured layer on the support side of the element. The cover sheetof the radiation sensitive element is then removed from the front side,and the film containing the mask image is then brought into air-freecontact with the radiation sensitive element on the front side. Theassembly may then be exposed to a UV radiation through the filmcontaining the mask image for about 14 minutes. The mask imageinformation is thus transferred to the flexographic plate precursor.

Relief Image Development

The carrier sheet including the mask, or the carrier sheet without themask, is then removed, by any appropriate means, such as peeling. Theexposed element (or imaged element), is then generally developed with asuitable developer to form a relief image. Development serves to removethe uncured regions of the radiation-sensitive element, leaving thecured regions that define the relief image on the substrate.

Any known developer for the imaged element can be used in thisprocessing step including those containing chlorinated organic solvents.Some useful developers are predominantly non-chlorinated organicsolvents. By “predominantly”, we mean that more than 50% (by volume) ofthe developer comprises one or more non-chlorinated organic solventssuch as aliphatic hydrocarbons and long chain alcohols (that is alcoholswith at least 7 carbon atoms). The remainder of the solutions can bechlorinated organic solvents, but typically, the chlorinated organicsolvents comprise less than 50% (by volume) of the developer.

Thus certain useful developers are predominantly what are known as“perchloroethylene alternative solvents” (PAS). These PAS are generallyvolatile organic compounds typically comprised of mixtures of aliphatichydrocarbons and long-chain alcohols. They are generally stable undernormal room temperature and storage conditions. Examples of suchcommercially available solvents include but are not limited to,PLATESOLV available from Hydrite Chemical Co. (Brookfield, Wis.),NYLOSOLV® available from BASF (Germany), FLEXOSOL® available from DuPont(Wilmington, Del.), OptiSol® available from DuPont (Wilmington, Del.),and SOLVIT® QD available from MacDermid (Denver, Colo.).

Development is usually carried out under conventional conditions such asfor from about 5 to about 20 minutes and at from about 23 to about 32°C. The type of developing apparatus and specific developer that are usedwill dictate the specific development conditions.

Post-development processing of the relief image may be suitable undersome circumstances. Typical post-development processing includes dryingthe relief image to remove any excess solvent ad post-curing by exposingthe relief image to curing radiation to cause further hardening orcrosslinking. The conditions for these processes are well known to thoseskilled in the art. For example, the relief image may be blotted orwiped dry, or dried in a forced air or infrared oven. Drying times andtemperatures would be apparent to a skilled artisan. Post-curing may becarried out using the same type of radiation previously used to exposethrough the mask image.

Detackification (or “light finishing”) may be used if the relief imagesurface is still tacky. Such treatments, for example, by treatment withbromide or chlorine solutions or exposure to UV or visible radiation,are well known to a skilled artisan.

The resulting relief image may have a depth of from about 2 to about 40%of the original thickness of the radiation-sensitive element imageablelayer. For a flexographic printing plate, the depth of the relief imagemay be from about 150 to about 500 μm. For a printed circuit board, theimageable layer is completely removed in either the exposed ornon-exposed regions, to reveal the metal layer underneath. Thus, in suchelements, the depth of the relief image depends upon the thickness ofthe imageable layer. Advantageously, the relief image has predominantlyshoulder angles of greater than 50°.

Development may also be possible by the thermal process as disclosed inU.S. Pat. No. 5,175,072 (Martens), U.S. Pat. No. 5,279,697 (Peterson etal.), and U.S. Pat. No. 6,998,218 (Markhart).

The following examples illustrate the practice of this invention but theinvention is not to be limited thereby.

EXAMPLES

The following materials and methods were used in the examples:

AIRVOL® 205 premix solution is a 10% solids aqueous solution of apoly(vinyl alcohol) that can be obtained from Air Products (Allentown,Pa.).

BUTVAR® B-76 is a poly(vinyl butyral) resin that can be obtained fromSolutia, Inc. (St. Louis, Mo.).

Byk® 333 is a polyether modified polydimethylsiloxane that can beobtained from Byk Chemie (Wallingford, Conn.).

Curcumin is a yellow dye that can be obtained from Cayman Chemicals (AnnArbor, Minn.).

Dyneon™ FC 2211 and 2178 are fluoroelastomers that can be obtained from3M Company (St. Paul, Minn.).

EMAX is a 60:40 copolymer of ethyl methacrylate and methacrylic acidthat can be obtained from Eastman Kodak Company (Rochester, N.Y.).

Fluon® AD1 is a PTFE dispersion that can be obtained from Asahi GlassFluoropolymers USA.

IR Dye A is an IR absorbing dye having the following structure and wasobtained from Eastman Kodak Company (Rochester, N.Y.).

MEK represents methyl ethyl ketone.

MIBK represents methyl iso-butyl ketone.

NeoRez 322 polyurethane resin that can be obtained from DSM NeoResins(Wilmington, Mass.).

NeoRez U395 is a polyurethane resin that can be obtained from DSMNeoResins (Wilmington, Mass.).

PCA represents a mixture of 70% (weight) poly(methyl cyanoacrylate) and30% (weight) poly(ethyl cyanoacrylate) as a 10% total solids solution in50/50 cyclopentanone/acetone, obtained from Eastman Kodak Company(Rochester, N.Y.).

Sudan Black is a black dye that can be obtained from Aldrich ChemicalsCo. (Milwaukee, Wis.).

Surfynol® FS-80 is a wetting agent that can be obtained from AirProducts & Chemicals, Inc. (Allentown, Pa.).

UVINUL® 3050 is an ultraviolet radiation absorbing dye that can beobtained from BASF (Germany).

Invention Examples 1 & 2 and Comparative Example 1

Two films of the present invention were prepared in the followingmanner.

A carrier sheet, formed of a 0.01 cm thick poly(ethylene terephthalate),was coated with the transparent layer formulation comprising Dyneon™ FC2211 (Invention Example 1) or Dyneon™ FC 2178 (Invention Example 2) outof MEK using a #12 wire rod to provide transparent layer having a drycoverage of 562 mg/m² when dried for 2 minutes at 93° C. This layer hasa refractive index at 400 nm of about 1.40 that is less than therefractive index of the carrier sheet, which is (at 400 nm) about 1.65.

Onto this transparent layer was coated a intermediate layer formulationcontaining Airvol® 205 poly(vinyl alcohol) out of an 80:20water:n-propanol mixture using a #10 wound-wire coating rod. Theresulting coating was dried at 2 minutes at 93° C. to provide a drycoating coverage of about 648 mg/m².

A barrier layer formulation was formed with the components and coatingsolvents listed in the following TABLE I and applied to the driedintermediate layer using a #10 wound-wire coating rod. The resultingcoating was dried at about 93° C. for 2 minutes to form a barrier layerto provide a coating coverage of about 378 mg/m².

TABLE I Barrier Layer Formulation Amount Formulation Component (%solids) PCA 84 NeoRez U395  5 IR Dye A 11 Acetone 40 partsCyclopentanone 60 parts

On the dried barrier layer, an imageable layer was formed using thecomponents and coating solvents shown in the following TABLE II using a#20 wound-wire coating rod. The resulting coatings were dried at about93° C. for 2 minutes to form imageable layers on the barrier layer at acoating coverage of about 1.51 g/m².

TABLE II Imageable Layer Formulation Component Formulation Amount (%solids) Sudan Black 10 UVINUL ® 3050 14.3 Curcumin 28.7 Nitrocellulose16 NeoRez U395 8.8 NeoRez U322 8.8 IR Dye A 13.5 MEK  5 partsCyclohexanone  5 parts MIBK 80 parts Ethanol 10 parts

The overcoat was formed using the components and coating solvents shownin the following TABLE III was applied over the dried imageable layerusing a #20 wound-wire coating rod. The resulting coatings were dried atabout 93° C. for 2 minutes to an overcoat at a coating coverage of about120 mg/m².

TABLE III Overcoat Formulation Formulation Amount Component (% solids)EMAX 61.5 Fluon ® AD1 10.5 Byk ® 333 30 Surfynol ® FS-80 10 Airvol ® 20515 NeoRez U322 8.8 Water 80 parts Ethanol 20 parts

The resulting films of this invention (Invention Examples 1 and 2) wereused to prepare flexographic printing plates in the following manner.

Each film was imaged on a Kodak Trendsetter® 800 Imager (KodakSQUARESPOT head, 830 nm exposure wavelength) to form a mask image. Themask image was then transferred from the imaged film by laminating it byapplying pressure (without heat) to a FLEXEL flexographic printing plateprecursor, that did not have a separation layer or anti-tack layer)available from Eastman Kodak Company (Rochester, N.Y.), so that theinterface between the imaged film and precursor was air-free.

The assembly masking film and flexographic printing plate precursor wereexposed through the career sheet to curing ultraviolet radiation using aKelleigh Model 310 Platemaker for 10 minutes and developed usingOptisol™ developer (available from Hydrite Chemical Co., LaCrosse,Wis.), followed by normal drying and post curing to provide imagedflexographic printing plates.

In Comparative Example 1, a method similar to the above as described forexamples 1 and 2, was used to prepare flexographic printing plates usinga masking film as in Invention Examples 1 and 2, but that did not havethe low refractive index layer

FIG. 2 shows the results in the resulting relief images as relief heightversus distance from the edge obtained for the three resultingflexographic printing plates prepared for Invention Examples 1 (Curve A)and 2 (Curve B) and Comparative Examples 1 (Curve C).

The results are also provided in the following TABLE IV.

TABLE IV Depth of Relief Depth Main UV Dot retention 0.40 mm (non-imagedExposure in 1% tints reversed lines Example areas) (mm) (min) (%) (μm)Invention 0.61 10 98 159 Example 1 Invention 0.63 10 96 178 Example 2Comparative 0.64 10 98 58 Example 1

In Invention Examples 1 and 2, the average shoulder angles were 55°while the average shoulder angle in Comparative Example 1 was about 25°.

These results can also be seen in FIGS. 3 a and 3 b showingcross-sections of a 380 μm reversed line in the resulting flexographicprinting plates for Comparative Example 1 (FIG. 3 a) and InventionExample 1 (FIG. 3 b).

Invention Example 3

Transparent layer formulations were prepared using the components shownin the following TABLE V and then used to prepare film samples.

TABLE V Film Sample 1 Film Sample 2 Film Sample 3 FormulationFormulation (% Formulation (% Formulation (% Component solids) solids)solids) Airvol ® 205 90 84 78 polyvinyl alcohol PEG 600 10 10 10polyethylene glycol NaBF₄  0  6 12 n-Propanol 20 parts 20 parts 20 partsWater 80 parts 80 parts 80 parts PEG 600 and NaBF₄ are available fromAldrich Chemical Company.

Each of the film samples above were prepared by coating the transparentlayer formulations shown in TABLE V and onto 4 mil (0.01 cm)poly(ethylene terephthalate) support (carrier sheet) using a #26wound-wire coating rod. The resulting transparent layers were dried at93° C. for 2 minutes to provide a dry coverage of about 3.5 g/m². Fullyfunctional film constructions were prepared by coating and drying oneach of these transparent layers: (i) barrier, (ii) imageable, and (iii)overcoat layers as described for Invention Example 1. After drying, eachfilm was imaged on a Kodak Trendsetter® 800 Imager (Kodak SQUARESPOThead, 830 nm exposure wavelength) to form a mask image. The mask imagewas then transferred from the imaged film by laminating it by applyingpressure (without heat) to a FLEXEL flexographic printing plateprecursor (that did not have a separation layer or anti-tack layer)available from Eastman Kodak Company (Rochester, N.Y.), so that theinterface between the imaged film and precursor was air-free (that is,optical contact was achieved).

The assembled masking film and flexographic printing plate precursorwere exposed through the career sheet to curing ultraviolet radiationusing a Kelleigh Model 310 Platemaker for 10 minutes and developed inOptisol developer followed by normal drying and post curing to provideimaged flexographic printing plates.

Table VI below lists the refractive index (“RI” at 400 nm) of eachtransparent layer described above, the calculated critical angle forinternal reflection, and the depths (“RLD”) of 0.50 mm wide reversedlines in the final printing plates made using the three films (maskimage constructions). The UV exposures were all 10 minutes.

TABLE VI Sample Relief depth Main UV Transparent Critical RLD (Film #)(mm) (min.) Layer RI Angle (μm) 1 0.77 10 1.62 80° 78 2 0.77 10 1.57 72°100 3 0.77 10 1.51 66° 126

As shown in TABLE VI, there is a strong relationship between therefractive index (RI) of the transparent layer and the depth of 500micron (μm) reversed lines in the finished plates. This relationship is(in this specific case and over this range of RI values:

RLD=785−436×(RI) R̂2 or (R)²=100%

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. A film comprising: a transparent carrier sheet and having thereon atleast one non-silver halide thermally sensitive imageable layer and acolorant disposed in said imageable layer or in a different layerbetween said carrier sheet and said imageable layer, an infraredradiation absorbing compound dispersed in a polymeric binder, and saidfilm further comprising a transparent layer disposed between saidcarrier sheet and said imageable layer and between said carrier sheetand said layer containing said colorant, which transparent layer has arefractive index lower than the refractive index of said carrier sheetor the refractive index of any optional immediately adjacent layerbetween said carrier sheet and said transparent layer.
 2. The film ofclaim 1 wherein said transparent layer has a refractive index at least0.04 lower than the refractive index of said carrier sheet.
 3. The filmof claim 1 wherein said transparent layer has a refractive index atleast 0.08 lower than the refractive index of each of said carrier sheetand said imageable layer, and said transparent layer optionally furthercomprises an adhesion promoter.
 4. The film of claim 1 wherein saidtransparent layer comprises a fluoroelastomer.
 5. The film of claim 1wherein said transparent layer has a thickness of from about 0.25 toabout 10 μm.
 6. The film of claim 1 further comprising an intermediatelayer disposed between said imageable layer and said transparent layer,said release layer having a thickness of from about 0.1 to about 10 μm.7. The film of claim 6 wherein said intermediate layer comprises apoly(vinyl alcohol), a cellulosic polymer, or hydrolyzed styrene maleicanhydride, and an adhesion promoter.
 8. The film of claim 1 wherein saidinfrared radiation absorbing compound is present in a layer other thansaid thermally sensitive imageable layer and is dispersed within apolymeric binder.
 9. The film of claim 1 wherein said infrared radiationabsorbing compound is an IR dye and said colorant is a UV-absorbing dye.10. The film of claim 1 further comprising a barrier layer disposedbetween said imageable layer and said carrier sheet, said barrier layeroptionally including an infrared absorbing compound.
 11. The film ofclaim 10 wherein said barrier layer comprises a heat-combustible polymerbinder.
 12. The film of claim 1 wherein said barrier layer is a metal ormetallized layer.
 13. The film of claim 1 comprising on said carriersheet, in order: a) said transparent layer comprising a fluoroelastomerand having a thickness of from about 0.25 to about 10 μm, saidtransparent layer optionally including an adhesion promoter, b) anintermediate layer comprising a poly(vinyl alcohol) and having athickness of from about 0.1 to about 10 μm, said intermediate layercomprising a colorant and optionally including an adhesion promoter, c)a barrier layer comprising a poly(cyanoacrylate) and an infraredradiation absorbing dye, d) said imageable layer comprising an infraredradiation absorbing dye and a UV-absorbing colorant dispersed in apolyurethane binder, and e) a poly(vinyl alcohol) overcoat, saidtransparent layer having a refractive index at least 0.08 lower than therefractive index of said carrier sheet.
 14. A method of making a reliefimage, the method comprising: A) forming a mask image by forming exposedand non-exposed regions in an imaged film, which film prior to imaging,comprises: a transparent carrier sheet having thereon at least onenon-silver halide thermally sensitive imageable layer and a colorantdisposed in said imageable layer or in a different layer between saidcarrier sheet and said imageable layer, an infrared radiation absorbingcompound dispersed in a polymeric binder, and a transparent layerdisposed between said carrier sheet and said imageable layer, whichtransparent layer has a refractive index lower than the refractive indexof said carrier sheet or the refractive index of any optionalimmediately adjacent layer between said carrier sheet and saidtransparent layer, B) transferring said mask image in said imaged filmto a radiation-sensitive element while there complete optical contactbetween said mask image and said radiation-sensitive element, C)exposing said radiation-sensitive element to curing radiation throughsaid mask image to form an imaged element, wherein said mask image isopaque to said curing radiation, and D) developing said imaged elementto form a relief image.
 15. The method of claim 14 wherein said imagedfilm is removed from optical contact with said exposedradiation-sensitive element between steps C and D.
 16. The method ofclaim 14 wherein said carrier sheet and transparent layer are removedfrom said imaged film between steps C and D.
 17. The method of claim 14wherein said radiation-sensitive element is a UV-sensitive element thatis exposed to UV radiation through said mask image to form an imagedelement, wherein said mask image is opaque to UV radiation.
 18. Themethod of claim 17 wherein said UV-sensitive element is a flexographicprinting plate precursor and said imaged element is a flexographicprinting plate.
 19. The method of claim 14 wherein the relief imageformed in said radiation-sensitive element has an average shoulder angleof at least 50°.
 20. The method of claim 14 wherein transferring step Bis carried out by lamination.
 21. The method of claim 14 wherein saidtransferring step B is carried out by applying a radiation-sensitivelayer while in liquid form, to said imaged film, and then forming saidradiation-sensitive element.
 22. The method of claim 14 wherein saidfilm comprises on said carrier sheet, in order: a) said transparentlayer comprising a fluoroelastomer that is a fluorinated acrylatepolymer, fluoroolefin, or mixtures thereof, said fluoroelasteromerhaving a thickness of from about 0.25 to about 10 μm, said transparentlayer optionally including an adhesion promoter, b) an intermediatelayer comprising a poly(vinyl alcohol) and having a thickness of fromabout 0.1 to about 10 μm, said intermediate layer comprising a colorantand optionally including an adhesion promoter, c) a barrier layercomprising a poly(cyanoacrylate) and an infrared radiation absorbingdye, d) said imageable layer comprising an infrared radiation absorbingdye and a UV-absorbing colorant dispersed in a polyurethane binder, ande) a poly(vinyl alcohol) overcoat, said transparent layer having arefractive index at least 0.08 lower than the refractive index of saidcarrier sheet.